Evidence of porphyry copper-type hydrothermal activity from a submerged remnant back-arc volcano of the Izu-Bonin arc Implications for the volcanotectonic history of back-arc seamounts

摘要

The first example of a porphyry copper-type hydrothermal activity associated with the back-arc volcanism in the Izu-Bonin arc is presented. Hydrothermally-altered rocks exposed at the flat summit of the Manji Seamount show strong similarities to those associated with porphyry copper-type deposits, i.e. (1) alteration mineral assemblages corresponding to potassic and propylitic alteration zone, (2) occurrence of stockwork of quartz-magnetite veinlets, (3) copper mineralization, (4) occurrence of fluid inclusions trapping high temperature (ca. 600 ℃) and hyper-saline (ca. 63 wt% NaCl equivalent) fluid, (5) occurrence of acidic plutonic rocks (e.g. tonalite porphyry), which are often genetically associated with prophyry copper deposits. Volcanism of Manji Seamount occurred at around 7 Ma and shares common chemical characteristics with other back-arc seamounts in the Izu-Bonin arc, i.e. more calc-alkaline affinity, enrichment in large ion lithophile elements (LILE) and high-field strength elements (HFSE) compared to those of volcanic front. The plutonic rocks are almost contemporaneous with the volcanic rocks and are regarded as comagmatic. The age of the volcanism is within the age range of other back-arc seamounts (17-3 Ma). The porphyry copper-type hydrothermal activity is presumed to have been driven by the volcanism in the back-arc during the period between the cessation of the back-arc spreading of the Shikoku Basin and the initiation of currently active rifting. The seamount was exposed above sea level when active, but was later deeply eroded down to the interior of the volcano. Subsequent subsidence and erosion produced a wave-cut flat summit and consequently the porphyry copper system outcrops at 700 m water depth. Some characteristics of the hydrothermal activity at Manji Seamount are shared by the porphyry copper deposits in the oceanic arcs in the southwestern Pacific, e.g. petrography and chemical characteristics of related igneous rocks. However, the hydrothermal activity at Manji Seamount is located further away from the volcanic front than the deposits in the southwestern Pacific, and regional back-arc volcanism is extensive and voluminous in the Izu-Bonin arc. The porphyry copper hydrothermal system reported here is a well-preserved example associates with voluminous back-arc volcanism (i.e. arc volcanism behind the volcanic front which is not associated with back-arc rifting) in an oceanic arc setting. The discovery of porphyry copper-type hydrothermal activity associated with extensive back-arc volcanism in an intra-oceanic arc implies that back-arc regions of ancient oceanic arcs which are exposed on land might host more porphyry copper deposits.